The present invention relates generally to optical waveguides and, more specifically, to a method of altering physical or optical characteristics of an optical waveguide or optical substrate and resulting apparatuses.
As telecommunication systems grow in customer base and services, the need for bandwidth has forced many component manufacturers and system designers to increase the number of communication channels or wavelength channels traveling in optical fibers while simultaneously decreasing the wavelength spacing between these channels. As designers attempt to meet these demands, they must still ensure channel isolation and uniformity. Adding even further design constraints, wavelength selective devices, such as those used in Dense Wavelength Division Multiplexing/Demultiplexing (DWDM) and channel Add/Drop Multiplexing (ADM) systems, must maintain strict adherence to an International Telecommunications Union channel wavelength standard, the so-called ITU grid. The difficulty with meeting the demands of the marketplace while concurrently satisfying industry standards has been quite detrimental to component manufacturers. Indeed, increasing channel capacity under existing rigid industry standards, like the Telecordia certification, has greatly compromised manufacturing yields for some of the industry""s most promising DWDM devices, i.e., arrayed waveguide gratings (AWG) and cascaded planar waveguide interferometers (CPWI) such as channel interleavers and slicers.
In terms of scalable channel capacity, function integration, and uniformity, AWG""s and CPWI""s offer greater potential for DWDM systems than thin film filters or fiber (waveguide) Bragg gratings. However, while AWG and CPWI devices have shown potential in DWDM test beds, the manufacturing yields for such structures are marginal at best making them very expensive to produce. These devices are lithographically fabricated chips of silicon on isolator (ISO) or silica on silicon (SOS) planar waveguide circuits.
The problems affecting AWG and CPWI devices are readily understandable given their operation and function. AWG""s and CPWI""s take in a multi-channel input signal and spatially separate out all, or a subset, of the various wavelengths, a technique also termed channel separation. These devices affect channel separation by creating a wavelength dependent interference pattern from derived replicas of the input signal propagating down two or more arrayed waveguides, each waveguide having a well defined optical path length. At the output, the positioning of the separated channels and the separation between adjacent channels is dependent upon both signal recombination and the phase differential acquired between the replica signals traveling through the different waveguides. Therefore, proper device operation is dependent upon the differences in optical path length, induced phase shift and both the absolute and relative signal amplitude between these waveguides. For example, if waveguides have even slightly incorrect optical path lengths, they may induce the wrong phase shifts on the propagating signals resulting in an offset between the measured channel wavelength and the ITU grid. Alternatively, an imbalance in the initial signal level between the waveguides (unbalanced signal loading) in AWG""s and CPWI""s will compromise channel isolation and uniformity. These problems are accelerated as the number of waveguides in a device increases.
The optical path length difference or phase difference is not only a function of differences in physical length between waveguides, but also differences in the index of refraction between waveguides. In fact, index of refraction variation throughout the substrate or bulk optical material is a substantial limitation to many multi-waveguide structures, because it has heretofore been generally difficult to accurately identify where the variations occur. Furthermore, the index of refraction is temperature dependent, and since AWG and CPWI devices, and other multiple waveguide structures for that matter, are to function in numerous field conditions variations in index of refraction can be exacerbated under temperature changes. For example, an AWG device could be minimally within industry standards at one temperature and fail those same standards when operated at another temperature. While it is always desirable to minimize the affects of temperature on device operation, such minimization is extremely difficult when concerned with waveguides having even slightly different indexes of refraction. Further failures due to errors acquired in the signal phase and amplitude of DWDM interference-based devices can arise from any polarization dependence. This dependence can affect both the signal throughput as in polarization dependent loss (PDL) and the wavelength response of the signal throughput as in a polarization dependent frequency (PDF). Also the unbalanced loss between waveguides (unbalanced channel loss) can result in device failure.
Based on the foregoing, there is a need for a method of correcting or changing the index of refraction within an optical waveguide in a spatially controlled manner and to do so in a procedure amenable to a post processing, quality control stage of device manufacture. There is also a need for a technique that corrects defects in known waveguide structures, defects like optical path length errors, unbalanced signal loading, PDL, PDF, unbalanced channel loss, and others. Further, there is a need for optical devices that exhibit such corrected defects.
In accordance with an embodiment, provided is a method of coupling a first waveguide and a second waveguide, wherein at least one of the first and second waveguides exists within an optical medium. The method includes the step of generating ultra-short laser pulses. Further, the method includes the step of applying a signal to the first waveguide and the second waveguide for measuring an output derived from the first waveguide and the second waveguide, the output being indicative of a performance metric of the first waveguide and the second waveguide. The method also includes the steps of, in response to the measured output, directing the ultra-short laser pulses within the bulk of the optical medium to write a facilitator segment such that the facilitator segment is within an evanescent coupling region of the first waveguide and within an evanescent coupling region of the second waveguide.
In another embodiment, provided is a method of coupling a first waveguide and a second waveguide, wherein at least one of the first and second waveguides exists within an optical medium. The method includes the step of generating ultra-short laser pulses. Further, the method includes the step of applying a signal to the first waveguide and the second waveguide for measuring an output derived from the first waveguide and the second waveguide, the output being indicative of a performance metric of the first waveguide and the second waveguide. The method also includes the steps of, in response to the measured output, directing the ultra-short laser pulses within the bulk of the optical medium to write a facilitator segment such that the facilitator segment is within an evanescent coupling region of the first waveguide and within an evanescent coupling region of the second waveguide.
In yet another embodiment, provided is a method of forming a graduated change in the index of refraction of a waveguide. The method comprises the steps of generating ultra-short laser pulses and scanning the ultra-short laser pulses over overlapping segments within the waveguide to form a desired axial index of refraction profile within the waveguide. In some of these embodiments, the axial index of refraction profile is a graded, step-wise, or corrugated profile.
In a further embodiment, provided is a method of repairing a defective waveguide comprising the steps of (A) applying an input signal to the defective waveguide and (B) detecting an output signal from the defective waveguide, the output signal being generated by the input signal. Additionally, the method includes the steps of (C) selecting a location associated with the defective waveguide and (D) applying a focused ultra-short laser pulse to the selected location to modify a characteristic of the waveguide. The method also includes the step of repeating at least some of steps (A)-(D) until the output signal from the defective waveguide indicates that the defective waveguide has been repaired.
In accordance with another embodiment, an optical device is provided having an optical medium; a first waveguide disposed in-bulk in the optical medium and having a cross-sectional refractive index profile; and a second waveguide disposed in-bulk in the optical medium and having a cross-sectional refractive index profile. A signal is transmitted in the optical device in a propagation direction, wherein the cross-sectional refractive index profile of the second waveguide is selectively varied along the propagation direction to cooperate with the cross-sectional refractive index profile of the first waveguide to produce a desired output of the optical device.
In accordance with another embodiment, provided is an optical device having an optical medium and a waveguide having a first segment and a second segment wherein the first and second segments are disposed in-bulk in the optical medium and have a respective cross-sectional refractive index profile. The cross-sectional refractive index profile of the first segment differs from the cross-sectional refractive index profile of the second segment such that the waveguide is polarization-selective.